Unit 1: Biol 235 Chapter 8

What is the value for the resting membrane potential for most neurons?

-70 mV The resting membrane potential for neurons depends on the distribution of both Na+ and K+ across the cell membrane. The potential is closer to the equilibrium potential of K+ because the cell is more permeable to K+.

Which of the following comprise a typical chemical synapse? 1. axon terminal 2. presynaptic cell 3. synaptic cleft 4. postsynaptic cell 5. dendrite

1, 2, 3, 4

Graded potentials may...

1. depolarize the membrane to the threshold voltage. 2. hyperpolarize the membrane. 3. be called EPSPs or IPSPs. 4. initiate an action potential.

Glial cells

1. guide neurons during growth and repair. 2. help maintain homeostasis of the brain's extracellular fluid. 3. provide structural and metabolic support. 4. provide structural and metabolic support and help maintain homeostasis of the brain's extracellular fluid.

What is the magnitude (amplitude) of an action potential?

100 mV The membrane goes from -70 mV to +30 mV. Thus, during the action potential, the inside of the cell becomes more positive than the outside of the cell.

Identify the FALSE statement. A. IPSPs depolarize the membrane. B. EPSPs that reach threshold can initiate an action potential. C. The trigger zone is the integrating center of the neuron. D. All of the above are true.

A

How is an action potential propagated along an axon?

An influx of sodium ions from the current action potential depolarizes the adjacent area. The influx of sodium ions depolarizes adjacent areas, causing the membrane to reach threshold and cause an action potential. Thus, the action potential is regenerated at each new area.

What type of conduction takes place in unmyelinated axons?

Continuous conduction An action potential is conducted continuously along an unmyelinated axon from its initial segment to the axon terminals. The term continuous refers to the fact that the action potential is regenerated when voltage-gated Na+‎ channels open in every consecutive segment of the axon, not at nodes of Ranvier.

Choose the FALSE statement. A. A hyperpolarized cell has a negative membrane potential compared to one at rest. B. Within a single neuron, different types of voltage-gated ion channels may have different threshold voltages. C. If a second graded potential begins before the first one ends, the voltage changes add together. D. Because sodium and potassium ions cross the membrane during an action potential, their concentrations inside the cell change significantly.

D

Once the stimulus alters the receptor on the cell's membrane, what happens next? A. A second messenger is activated on the inside of the cell. B. Ion channels open, allowing ions to enter or exit. C. The membrane permeability is altered. D. Any of the above could happen next.

D

Which is NOT considered a neuron of the efferent pathway? A. parasympathetic B. somatic motor C. sympathetic D. sensory

D

Choose all the items that are incorrectly matched. A. activation gate - opens during depolarization B. inactivation gate - opens during repolarization C. activation gate - closed at rest D. inactivation gate - open at rest E. All of the above are correctly matched.

E

When two or more graded potentials arrive at the trigger zone, which of the following could NOT happen? A. Two inhibitory stimuli may be additive, resulting in lower excitability. B. An excitatory and inhibitory signal can cancel each other out. C. Two excitatory stimuli may be additive, and summation could occur. D. An excitatory and inhibitory signal can cancel each other out; two excitatory stimuli may be additive, and summation could occur. E. An excitatory and inhibitory signal can cancel each other out; two excitatory stimuli may be additive, resulting in lower excitability and summation could occur.

E

What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?

Inactivation gates of voltage-gated Na+‎ channels close, while activation gates of voltage-gated K+‎ channels open. Closing of voltage-gated channels is time dependent. Typically, the inactivation gates of voltage-gated Na+‎ channels close about a millisecond after the activation gates open. At the same time, the activation gates of voltage-gated K+‎ channels open.

Which of the following will increase the conduction rate of action potentials?

Increase the diameter of the axon, increase the resistance of the axon membrane to ion leakage.

Where do most action potentials originate?

Initial segment The first part of the axon is known as the initial segment. The initial segment is adjacent to the tapered end of the cell body, known as the axon hillock.

The falling phase of the action potential is due primarily to

K+ flow out of the cell.

The major determinant of the resting potential of all cells is

K+ gradient between the cell and interstitial fluid.

The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.

K+; Na+ More K+ moves out of the cell than Na+ moves into the cell, helping to establish a negative resting membrane potential.

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?

Leak channels Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.

In which type of axon will velocity of action potential conduction be the fastest?

Myelinated axons with the largest diameter The large diameter facilitates the flow of depolarizing current through the cytoplasm. The myelin sheath insulates the axons and prevents current from leaking across the plasma membrane.

Excitatory neurotransmitters of the CNS usually act by opening __________ channels.

Na+

The concentrations of which two ions are highest outside the cell.

Na+ and Cl- Both Na+ and Cl- are in higher concentrations outside the cell.

The rising phase of the action potential is due to

Na+ flow into the cell.

The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?

Na+ is pumped out of the cell and K+ is pumped into the cell. Na+ is pumped out of the cell against its electrochemical gradient and K+ is pumped into the cell against its concentration gradient.

What prevents the Na+ and K+ gradients from dissipating?

Na+-K+ ATPase Also known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels.

What characterizes repolarization, the second phase of the action potential?

Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV. The plasma membrane was depolarized to a positive value at the peak of the first phase of the action potential. Thus, it must repolarize back to a negative value.

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?

Resting membrane potential (RMP) The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation.

Myelin is formed by

Schwann cells and oligodendrocytes.

Why does the action potential only move away from the cell body?

The areas that have had the action potential are refractory to a new action potential. Sodium channels are inactivated in the area that just had the action potential.

Why does regeneration of the action potential occur in one direction, rather than in two directions?

The inactivation gates of voltage-gated Na+‎ channels close in the node, or segment, that has just fired an action potential. At the peak of the depolarization phase of the action potential, the inactivation gates close. Thus, the voltage-gated Na+‎ channels become absolutely refractory to another depolarizing stimulus.

On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?

The inside surface of the plasma membrane is much more negatively charged than the outside surface. The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na+ and K+ gradients and the selective permeability of the membrane to Na+ and K+.

What characterizes depolarization, the first phase of the action potential?

The membrane potential changes from a negative value to a positive value. The plasma membrane, which was polarized to a negative value at the RMP, depolarizes to a positive value.

What event triggers the generation of an action potential?

The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV. This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.

What is the function of the myelin sheath?

The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals. The myelin sheath increases the velocity of conduction by two mechanisms. First, myelin insulates the axon, reducing the loss of depolarizing current across the plasma membrane. Second, the myelin insulation allows the voltage across the membrane to change much faster. Because of these two mechanisms, regeneration only needs to happen at the widely spaced nodes of Ranvier, so the action potential appears to jump.

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.

The presence of concentration gradients and leak channels The concentration gradient and the large number of K+ leak channels allow for rather robust K+ diffusion out of a cell. In contrast, the concentration gradient and the relatively few Na+ leak channels allow for much less Na+ diffusion into a cell.

The plasma membrane is much more permeable to K+ than to Na+. Why?

There are many more K+ leak channels than Na+ leak channels in the plasma membrane. More leak channels translates into more leakiness. Thus the outward flux of K+ is greater than the inward flux of Na+.

What opens first in response to a threshold stimulus?

Voltage-gated Na+ channels The activation gates of voltage-gated Na+ channels open, and Na+ diffuses into the cytoplasm.

What is the first change to occur in response to a threshold stimulus?

Voltage-gated Na+ channels change shape, and their activation gates open. The activation gates of voltage-gated Na+ channels open very rapidly in response to threshold stimuli. The activation gates of voltage-gated K+ channels are comparatively slow to open.

The velocity of the action potential is fastest in which of the following axons?

a small myelinated axon The myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Propagation along myelinated axons is known as saltatory conduction.

The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?

acetylcholine acetylcholine is broken down by acetylcholinesterase before being returned to the presynaptic neuron's axon terminal.

Temporal summation refers to

additional graded potential(s) arriving before previous ones have ceased.

The all-or-none principle states that

all stimuli great enough to bring the membrane to threshold will produce action potentials of identical magnitude.

Exocrine glands, smooth muscles, and cardiac muscles are controlled by the

autonomic nervous system.

Where in the neuron is an action potential initially generated?

axon hillock this region (first part of the axon) receives local signals (graded potentials) from the soma and dendrites and has a high concentration of voltage-gated Na+ channels.

Neurotransmitter is stored and released from

axon terminals and axon varicosities.

Action potentials occur in the membrane of

axons

The term hyperkalemia specifically indicates too much potassium in which fluid compartment?

blood

During depolarization, which gradient(s) move(s) Na+ into the cell?

both the electrical and chemical gradients A positive ion is driven into the cell because the inside of the cell is negative compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is greater outside the cell.

The ion necessary to initiate the release of acetylcholine into the synaptic cleft is

calcium

Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.

chemically gated; postsynaptic the neurotransmitter is a chemical released from the presynaptic membrane, so it would open chemically gated channels on the postsynaptic membrane.

Ion channel inactivation is

closing of the channel even when the stimulus continues.

Branches that sometimes occur along the length of an axon are called

collaterals

The pattern of synaptic connectivity where a large number of presynaptic neurons provide input to a single postsynaptic neuron, is known as:

convergence

The part of the neuron that receives most of the incoming signals is the ______.

dendrite

An excitatory postsynaptic potential (EPSP)

depolarizes a neuron, increasing the likelihood of an action potential.

An action potential is self-regenerating because __________.

depolarizing currents established by the influx of Na+‎ flow down the axon and trigger an action potential at the next segment The Na+‎ diffusing into the axon during the first phase of the action potential creates a depolarizing current that brings the next segment, or node, of the axon to threshold.

Binding of the neurotransmitter to its receptor causes the membrane to __________.

either depolarize or hyper polarize the neurotransmitter can cause the postsynaptic membrane to either depolarize or hyperpolarize, depending on which ion channels are opened.

The absolute refractory period of an action potential

ensures one-way travel down an axon, allows a neuron to ignore a second signal sent that closely follows the first, and prevents summation of action potentials.

Neurotransmitters are usually released into synapses by ________.

exocytosis

Voltage-regulated channels are located

in the membranes of axons.

The neurons of the central nervous system are also known as _______.

interneurons

When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,

ion channels in the plasma membrane of the receiving neuron open.

A stronger stimulus to a neuron results in ___________.

larger voltage changes in graded potentials and greater frequency of action potentials produced in response

In most cells, the concentration of the ions contributing to the membrane potential are ______ inside compared to outside, with the exception being _____.

lower; potassium

In order to signal a stronger stimulus, action potentials become

more frequent.

Spatial summation refers to

multiple graded potentials arriving at one location simultaneously.

A molecule that carries information across a synaptic cleft is a...

neurotransmitter Neurotransmitter molecules carry information across a synaptic cleft.

In response to binding a neurotransmitter, a postsynaptic cell can

open chemically gated ion channels, causing graded potentials known as fast synaptic potentials, close ion channels via G proteins and second messenger systems, producing slow responses, and regulate protein synthesis and affect the metabolic activities of the postsynaptic cell.

The point during an action potential when the inside of the cell has become more positive than the outside is known as the

overshoot

During the relative refractory period, __________ gates are open.

potassium

In a synapse, neurotransmitters are stored in vesicles located in the __________.

presynaptic neuron neurotransmitters are stored in the axon terminals of the presynaptic neuron.

The sodium-potassium exchange pump

requires ATP to function.

Inhibitory postsynaptic potentials (IPSPs)

result in local hyperpolarizations.

Hyperpolarization results from __________.

slow closing of voltage-gated K+ channels the slow closing of the voltage-gated K+ channels means that more K+ is leaving the cell, making it more negative inside.

Autonomic motor neurons are subdivided into the

sympathetic and parasympathetic divisions.

The region where the axon terminal meets its target cell is called the

synapse

The small space between the sending neuron and the receiving neuron is the

synaptic cleft. The synaptic cleft is the small space between the sending neuron and the receiving neuron.

When a second EPSP arrives at a single synapse before the effects of the first have disappeared, what occurs?

temporal summation

The repolarization phase of an action potential results from __________.

the opening of voltage-gated K+ channels as the voltage-gated K+ channels open, K+ rushes out of the cell, causing the membrane potential to become more negative on the inside, thus repolarizing the cell.

If a signal from a sending neuron makes the receiving neuron more negative inside,

the receiving neuron is less likely to generate an action potential. If the receiving neuron is more negative inside, it is less likely to generate an action potential.

The total amount of neurotransmitter released at the axon terminal is directly related to

the total number of action potentials.

Whether or not a neuron produces an action potential at a given moment depends on _______.

the total potential change reaching threshold voltage at the trigger zone

When calcium ions enter the synaptic terminal,

they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.

The site of information integration in the nervous system is the

trigger zone

Which of the following is the location where action potentials originate?

trigger zone

The resting membrane potential results from

uneven distribution of ions across the cell membrane and differences in membrane permeability to Na+ and K+.

An action potential releases neurotransmitter from a neuron by opening which of the following channels?

voltage-gated Ca2+ channels opening of these channels causes calcium to move into the axon terminal. Calcium inside the neuron causes the vesicles to merge with the membrane and release the neurotransmitter via exocytosis into the synaptic cleft.

The depolarization phase of an action potential results from the opening of which channels?

voltage-gated Na+ channels when the voltage-gated Na+ channels open, Na+ rushes into the cell causing depolarization.